Alkali metal borohydrides have been commercially recognized as powerful nucleophilic reducing agents. The first such borohydride to be recognized was lithium triethylborohydride, which has been found to reduce most organic functional groups in a matter of minutes under very mild conditions at room temperature. Other lithium mono- and di-alkylborohydrides have since been prepared and have been determined to have suitable reducing characteristics for various organic functional groups. One commercially available reducing agent in this category is lithium 9-boratabicyclo(3.3.1)nonane (hereinafter "Li9-BBNH"). This reagent is prepared by reacting lithium hydride with 9-BBN in tetrahydrofuran as a solvent. The reaction is extremely slow, taking 48 hours at 25.degree. C. In industry, this reaction is typically carried out at 65.degree. C. for a period of 24 hours.
The prolonged high temperature reflux required to produce such lithium mono- and di-alkylborohydrides greatly limits the structural range of products resulting. This is because mono- and di-alkylboranes readily disproportionate within hours at reflux temperatures.
In addition to requiring long preparation times and resulting in a small variety of products, such known lithium mono- and di-alkylborohydrides do not make efficient use of the metal hydride moiety included therein when used to reduce organic functional groups. For example, the reducing agent lithium triethylborohydride (LiEt.sub.3 BH) reacts with a reducible functional group to produce the byproduct triethylborane. This byproduct then reacts with the remaining lithium triethylborohydride to form LiEt.sub.3 BH:BEt.sub.3. This latter compound is relatively inactive. In effect, the reducing agent serves as its own inactivator and therefore requires at least a 100% excess for reduction. The conventional reducing agent Li9-BBNH behaves similarly, reacting with many organic compounds to liberate less reactive 9-BBN.
Finally, an additional drawback of such conventional lithium mono- and di-alkylborohydride reducing agents is that they must be inactivated prior to recovery of the desired reduction product, which usually entails an oxidative workup that many organic compounds can not tolerate.